| Ordermatic Carbon Reduction Ltd | Households | 2014 |
| Ordermatic Services Limited | Administrative & support | 2024 |
| Orelum Limited | Information & communication | 2026 |
| Oren Fortune Property LLP | | 2022 |
| Orfeur Timber Limited | Professional & scientific | 1915 |
| Organic Green Broker UK Ltd | Wholesale & retail | 2025 |
| Organic Pools Ltd | Administrative & support | 2015 |
| Organisational Coaching Ltd | Professional & scientific | 2022 |
| Organisational Interventions Ltd | Professional & scientific | 2018 |
| Organised Human Limited | Information & communication | 2023 |
| Organy Limited | Information & communication | 2017 |
| Oriental Rugs of Norwich Limited | Wholesale & retail | 2002 |
| Oriental Wellbeing & Healthcare Ltd | Health & social care | 2017 |
| Origa Fashion Ltd | Wholesale & retail | 2018 |
| Origen Advisors Limited | Professional & scientific | 2004 |
| Origin Woodworks Ltd | Manufacturing | 2023 |
| Original a K Kebab House Ltd | Hospitality | 2018 |
| Original G Sauce Limited | Manufacturing | 2023 |
| Original Holiday Homes Ltd | Real estate | 2022 |
| Original K Ltd | Wholesale & retail | 2022 |
| Original Peri Cottage Ltd | Hospitality | 2021 |
| Orion Air-Conditioning Ltd | Construction | 2023 |
| Orion Psychiatry Ltd | Health & social care | 2026 |
| Orion's Embrace CIC | Administrative & support | 2025 |
| Orleo Aviation Ltd | Transport & logistics | 2025 |
| Orme Mineral Services Ltd | Mining & quarrying | 2016 |
| Orme Trading Ltd | Agriculture & fishing | 2025 |
| Orourke Holdings (Essex) Limited | Financial services | 2024 |
| Orrick (UK) Limited | Construction | 1999 |
| Orrick Hemblington Limited | Construction | 2018 |
| Orrinex Ltd | Administrative & support | 2026 |
| Ortona Investments Limited | Financial services | 2012 |
| Ortona Management Limited | Real estate | 2012 |
| Ortona Properties Limited | Real estate | 2012 |
| Orun Inc Property Relocation Limited | Hospitality | 2021 |
| Orveon Global Ltd. | Financial services | 2021 |
| Orwelld Ltd | Other services | 2016 |
| OS Barbers Ltd | Other services | 2026 |
| Os-Carz Ltd | Wholesale & retail | 2022 |
| Osapiens UK Limited | Information & communication | 2024 |
| Osborne Acquisitions Ltd | Real estate | 2018 |
| Osborne Aviation Limited | Manufacturing | 2026 |
| Osborne Medical Services Ltd | Health & social care | 2022 |
| OSC Reporting Ltd | Health & social care | 2025 |
| Oscar Industries Ltd | Construction | 2011 |
| Oscar Percy Limited | Hospitality | 2014 |
| Oscarsiana Estates Limited | Professional & scientific | 2012 |
| Osfox Limited | Financial services | 2026 |
| OSG Homes Ltd | Real estate | 2025 |
| Osia Industries Limited | Wholesale & retail | 2026 |
| Osiris Technologies Ltd | Information & communication | 2018 |
| Osmed Ltd | Extraterritorial | 2013 |
| Osmotics Limited | Wholesale & retail | 2005 |
| Osoon Home Ltd | Wholesale & retail | 2015 |
| Osprey Research Ltd | Wholesale & retail | 2016 |
| Ossiano Capital UK Ltd | Financial services | 2025 |
| Osso Catering Limited | Hospitality | 2025 |
| Ostensio Limited | Real estate | 2016 |
| Osteostasis Ltd | Other services | 2016 |
| Osyris Event Services Limited | Other services | 2020 |
| OTF Solutions Ltd | Construction | 2022 |
| OTG Composites Engineering Ltd | Professional & scientific | 2023 |
| Other People's Sculpture Limited | Manufacturing | 2007 |
| Other Studio Norwich Limited | Other services | 2026 |
| Otherfolk Studio Ltd | Arts & recreation | 2026 |
| Otimo Partnerships Ltd | Administrative & support | 2017 |
| Otley Investments Limited | Real estate | 2016 |
| OTM Financial Ltd | Financial services | 2017 |
| Otoko Barbers Ltd | Other services | 2025 |
| OTS Electrical Limited | Construction | 2024 |
| Ottaway Properties Ltd | Real estate | 2018 |
| Otterplace Limited | Professional & scientific | 1992 |
| Otters.Ai Limited | Information & communication | 2021 |
| Ottimo Construction Ltd | Construction | 2021 |
| Otzi Ltd | Manufacturing | 2019 |
| OUR Little Play Home Ltd | Other services | 2023 |
| OUR Norfolk Ltd | Professional & scientific | 2025 |
| OUR Power Community Energy CIC | Professional & scientific | 2023 |
| OUR Social Collective Limited | Information & communication | 2017 |
| OUR Social Creative Limited | Professional & scientific | 2020 |
| OUR Social Management Limited | Professional & scientific | 2020 |
| OUR Success IS UN Limited | Agriculture & fishing | 2019 |
| Ourcom Ltd | Professional & scientific | 2024 |
| Ourhire Limited | Transport & logistics | 2007 |
| Ourotech Limited | Health & social care | 2019 |
| Ours Event Ltd | Hospitality | 2025 |
| OUT of Holdings Ltd | Extraterritorial | 2022 |
| OUT of Office Events Ltd | Administrative & support | 2010 |
| Out-And-Out Limited | Real estate | 2021 |
| Outaszide Ltd | Information & communication | 2025 |
| Outfox Marketing Limited | Professional & scientific | 2014 |
| Outlab Ltd | Professional & scientific | 2023 |
| Outlaw Record Studios Limited | Information & communication | 2013 |
| Outlook Global Media Limited | Extraterritorial | 2016 |
| Outlook Publishing Ltd | Information & communication | 2012 |
| Outmost Studio Limited | Information & communication | 2023 |
| Outout Events Ltd | Arts & recreation | 2020 |
| Outpost Advisors Limited | Professional & scientific | 2024 |
| Outrun Events Limited | Administrative & support | 2024 |
| Outshots Ltd | Wholesale & retail | 2026 |